This invention relates to an optical detector circuit for a photometric instrument. More particularly, this invention relates to an optical detector circuit providing a high precision A/D conversion of a detected optical signal with an improved signal to noise ratio and enhanced interference characteristics. Still more particularly, this invention relates to an optical detection circuit using dual slope integration through a programmed sequence comprising a discharge cycle wherein a reference signal and a true signal are discharged to a zero level, a zero blanking cycle through which an inverted blanking signal is integrated, a measuring cycle in which both the reference signal and the true signal are integrated, and a deintegration or counting cycle during which the ratio between the integrated reference signal and the integrated measured signal is digitally counted.
Photometric instruments are known to the art for providing an analog signal representative of an optical characteristic of a sample of interest to an investigator. Among such instruments are spectrophotometers, nephelometers, and fluorimeters. Such instruments provide an analog output signal which is digitally converted in an A/D converter for measurement of the magnitude of a light-related signal as an indicator of a parameter in a measured sample which is of interest.
A number of instruments are known which use dual slope integration techniques in an analog to digital conversion in which an unknown analog signal is integrated for a fixed time period and subsequently related to integration of a reference voltage wherein digital conversion occurs through pulses gated through a pulse counter.
In such devices, it is a consistent problem in the art to develop an instrument which exhibits superior noise and interference rejection characteristics. Accordingly, it is an object of the invention to provide an optical detector circuit which exhibits an improved signal-to-noise ratio through a technique of time integration of the true signal and a blank reference signal.
It is another object of this invention to provide an improved optical detector circuit with improved interference rejection characteristics by choosing an integration period as a precise multiple of the primary sources of interference, such as power line frequencies and fluorescent light frequencies.
It is still another object of this invention to provide a high precision A/D conversion of the integrated signal for a ratioed signal representative of a detected reference signal and a detected sample signal through a controlled program comprising a discharge cycle, a zero blanking cycle, a measuring cycle and a deintegration or counting cycle.
These and other objects of the invention will become apparent from the detailed description of the invention which follows in conjunction with the accompanying drawings.